1. Field of the Invention
This invention relates to drive circuits for stepper motor and in particular to switch mode current regulators for providing current to the coils of a stepper motor for use in peripheral devices such as electronic printing terminals.
2. Description of the Prior Art
Prior art stepper motors have had power supplies that were large enough to provide power required for peak acceleration and torque. This type of driver required a relatively large amount of power to operate, with attendant problems such as excessive weight and heat dissipation.
Switch mode current regulators were then provided as drivers for stepper motors. In these state-of-the-art regulators, it is common practice to apply a sawtooth voltage at the desired frequency to a comparator where it is compared to the control voltage. The output of the comparator is a pulse stream with the pulse widths dependent on the control voltage. The sawtooth voltage is generated by a sawtooth oscillator, or by capacitive integration of a square wave.
In the present invention, instead of generating a sawtooth voltage, the current through the motor coil itself is conducted through a sensing resistor with that voltage then applied directly to one input to a comparator as the ramping voltage necessary to achieve pulse width modulation.